Feedback control of exhaust gas recirculation based on combustion condition

ABSTRACT

To effect EGR in an internal combustion engine with the maintenance of adequate balance between the suppression of NO x  formation and preservation of stable engine operation, the condition of combustion in the engine is taken as the basis of feedback control of the volume of recirculated exhaust gas so as to correct a deviation of the total amount of the recirculated exhaust gas and unexhausted combustion gas in the combustion chamber from a desired amount. The intensity of an ionic current produced upon combustion in each combustion chamber, the magnitude of mechanical vibration of the engine or the frequency of misfire in a combustion chamber is detected as an exact indication of the condition of combustion.

This is a division, of application Ser. No. 825,130, filed Aug. 16, 1977now U.S. Pat. No. 4,186,701 of Feb. 5, 1980.

BACKGROUND OF THE INVENTION

This invention relates to a feedback control system for controlling therecirculation of exhaust gas through an internal combustion engine,which system has a combustion condition sensor to provide a feedbacksignal.

Concerning the prevention of air pollution by exhaust gas of internalcombustion engines, the recirculation of a portion of exhaust gas backinto the engine intake is probably the most widely employed techniquefor suppressing the emission of NO_(x) . The recirculation of exhaustgas (EGR) has the effect of lowering the maximum combustion temperaturein the engine combustion chambers so that the formation of NO_(x) in thecombustion chambers can be suppressed.

The suppressive effect of EGR on the formation of NO_(x) is enhanced asthe volume of the recirculated exhaust gas relative to the volume offresh air admitted into the engine is increased (this volume ratio willherein be referred to as EGR rate). To maintain NO_(x) emission below apermissible level, there is a need of effecting EGR at considerably highEGR rates. On the other hand, the employment of high EGR rates tends tocause instability of the engine operation. Since the recirculatedexhaust gas serves as an inert diluent to a combustible gas mixture, notonly the maximum combustion temperature but also the combustion pressurelowers as the EGR rate is enhanced. Accordingly the EGR rate should becontrolled in dependence on the engine operating condition so as tomaintain an adequate balance between the suppression of NO_(x) emissionand the preservation of a stable engine operation, and high precision isrequired of the control especially when high EGR rates are involved inthe scope of the control.

In conventional EGR control systems, it is a usual way of operting anEGR control valve to employ a vacuum-operated actuator which isconnected to the induction passage of the engine, so that the EGRcontrol valve is operated in dependence on the magnitude of vacuumeither at a venturi section of the induction passage or in theneighborhood of a throttle valve. In this type of control systems, thecontrol is accomplished in a programmed manner so as to regulate the EGRrate to a target value which is preset based on an assumed relationshipbetween the EGR rate or the aforementioned vacuum and the condition ofthe combustion in the engine.

The venturi section vacuum, for example, is of course an indication ofthe engine operating condition, but there is a limitation to theprecision in the control of EGR when the EGR control valve is directlyoperated by such vacuum. Furthermore, the rate of EGR in a programmedcontrol system chances to remain constant even though a substantialfluctuation occurs in an actual or realized EGR rate as a result ofchanges in the engine operating condition. Sometimes the operation ofthe engine temporarily loses stability from this reason. The instabilityof the engine operation resulting from a deviation of a realized EGRrate from an intended EGR rate is a matter of great concern particularlywhen EGR is effected up to a very high EGR rate, for example, about 40%,as in an engine system featuring "two-point ignition" in each combustionchamber, proposed by research workers of Nissan Motor.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an EGR controlsystem of a feedback control type for internal combustion engines.

It is another object of the invention to provide an EGR control systemfor internal combustion engines, which control system includes acombustion condition sensor to accomplish a feedback control of the rateof EGR based on the condition of combustion in the engine, that is, anexact indication of a realized EGR rate.

An EGR control system according to the invention comprises a flowcontrol valve to regulate the volume flow rate of the exhaust gas in anexhaust recirculation passage, a sensor means for sensing the conditionof combustion in a combustion chamber of the engine and producing afirst electrical signal representing the sensed combustion condition, acontrol circuit for producing a second electrical signal based on thefirst signal, which second signal indicates a decrease in theaforementioned volume flow rate of the exhaust gas when the first signalimplies that the condition of combustion approaches unstableness, and anactuator means for operating the control valve in response to the secondsignal.

This EGR control system is characterized in that the condition ofcombustion is taken as the basis of feedback control. The combustioncondition, typically represented by the rate of combustion, is affectedby the total volume of the recirculated exhaust gas and the unexhaustedcombustion gas in the combustion chamber relative to the volume of freshair-fuel mixture, so that the feedback control of the volume of therecirculated exhaust gas based on the combustion condition can maintaina stable combustion even when an abrupt change occurs in the engineoperating condition.

The condition of combustion can directly be detected by inserting aneedle-like probe into the combustion chamber so as to produce an ioniccurrent in the combustion chamber when combustion occurs therein sincethe intensity of the ionic current is proportional to the product of theion density and the rate of combustion.

The combustion condition can be detected also by measuring the magnitudeof mechanical vibration of the engine using a conventional vibrationpickup since the vibration intensifies as the combustion conditionapproaches unstableness as the result of an increase in theaforementioned total volume of the recirculated exhaust gas andunexhausted combustion gas.

Furthermore, it is also possible to detect the combustion condition bydetecting the frequency of misfire in the combustion chambers. This canbe accomplished by disposing a spark-gap in the exhaust passage at asection where exhaust gas retains a sufficiently high temperature anddetecting the frequency of sparking across the spark-gap.

The actuator means preferably include a vacuum-operated valve actuatorand an electromagnetic valve which regulates the magnitude of vacuumapplied to the actuator in response to the output of the controlcircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic presentation of an exhaust gas recirculationsystem as an embodiment of the invention;

FIG. 2 is an explanatory graph showing the relationship between thequantity of combustion gas retained in a combustion chamber and an ioniccurrent produced in the combustion chamber under a fixed dischargecondition;

FIG. 3 shows a modification of the control system of FIG. 1 as anotherembodiment of the invention;

FIG. 4 is a diagrammatic presentation of a still different EGR controlsystem also as an embodiment of the invention;

FIG. 5 shows the principle of a misfire sensor employed in the controlsystem of FIG. 4; and

FIG. 6 is a chart showing a variation in the output of the misfiresensor of FIG. 5.

DESCRIPTION OF PREFERRED EMBODIMENTS

An EGR control system shown in FIG. 1 as a first embodiment of theinvention is characterized in that a feedback signal representing thecondition of combustion in each combustion chamber 12 of an internalcombustion engine 10 is produced by measuring an ionic current producedin the combustion chamber 12.

A fuel system for this engine 10 has an induction passage 14 equippedwith a carburetor 16 and a throttle valve 18. The combustion chamber 12is defined in an engine cylinder above a piston 20 with the provision ofan intake valve 22, an exhaust valve 24 and a spark plug 26 in the usualmanner. An exhaust gas recirculation (EGR) passage 30 branches from anexhaust passage 28 to interconnect it to the induction passage 14down-stream of the throttle valve 18 for the purpose of recirculating aportion of the exhaust gas to the combustion chamber 12. A conventionalEGR control valve 32 is associated with the EGR passage 30 to controlthe volume flow rate of the exhaust gas through this passage 30. In theillustrated case, the control valve 32 has a tapered valve member 34axially movable in an orifice 36 formed in the passage 30. Avacuum-operated valve actuator 38 for moving the valve member 34 has adiaphragm 40 which holds the stem of the valve member 34 and serves as apartition between a vacuum chamber 42 and an atmospheric pressurechamber 44. A spring 46 is installed in the vacuum chamber 42 to biasthe diaphragm 40 towards the atmospheric pressure chamber 44. An intakevacuum produced by the operation of the engine 10 is applied to thevacuum chamber 42 through a conduit 48, and the valve member 34 isarranged such that an effective cross-sectional area of the orifice 36increases as the magnitude of vaccum applied to the chamber 42increases.

An electromagnetic valve 50 is associated with the vacuum transmissionconduit 48 to admit a variable quantity of air into this conduit 48through an air admission passage 52 and is operated by the output of anelectronic control circuit 54. A needle-like probe 56 is inserted intothe combustion chamber 12. On the outside of the engine 10, this probe56 is connected to a DC power source (not shown) such that a voltageV_(p) can be impressed across the probe 56 and the wall of thecombustion chamber 12. The output of the probe 56 is put into thecontrol circuit 54 via an ionic current detection circuit 58.

Upon combustion of a combustible gas mixture in the combustion chamber12, there occurs ionization of a portion of the combustion gas.Accordingly the impression of the voltage V_(p) on the probe 56 duringthe combustion causes a discharge across the probe 56 and the wall ofthe combustion chamber 12 with the production of an ionic current I. Theintensity of the ionic current I is given by the following generalformula:

    IαL·N.sub.i.sup.0.75 ·U.sup.0.75 ·V.sub.p.sup.0.5                                 (1)

where L is the length of the probe 56 protruded into the combustionchamber 12, N_(i) is the ion density, and U is the velocity of thecombustion gas relative to the probe 56 (the rate of combustion). Thedensity N_(i) and the rate of combustion U depend on the amount of thecombustion gas retained in the combustion chamber 12 at the start of thecombustion, that is, the total amount of the unexhausted combustion gasand the recirculated exhaust gas. Accordingly the ionic current Irepresents the rate of combustion or the condition of combustion.

FIG. 2 shows the dependence of the ionic current I on the aforementionedamount of the retained combustion gas. The ionic current I decreases asthe amount of the retained combustion gas increases.

It is possible, therefore, to know the proportion of the retainedcombustion gas to the fresh air-fuel mixture and estimate the rate ofcombustion by measuring the ionic current I. The detection circuit 58supplies a feedback signal representing the intensity of the ioniccurrent I to the control circuit 54. In the control circuit 54, thissignal is compared with a reference signal which implies a currentintensity corresponding to an intended amount of the retained combustiongas, that is, a desired EGR rate as the aim of the control. The outputof the control circuit 54 is a power signal for operating theelectromagnetic valve 50 and varies so as to cancel any deviation of theinput signal from the reference signal by regulating the admission ofair into the vacuum transmission conduit 48 through the electromagneticvalve 50. This means the regulation of the magnitude of vacuum appliedto the actuator 42 such that the opening area of the EGR control valve32 is varied until the realization of an intended EGR rate.

The control system may optionally comprise additional sensors (notshown) for utilizing some variables relating to the operating conditionof the engine 10, for example engine speed and/or intake vacuum, also asinputs to the control circuit 54 with the purpose of modifying theoutput of the circuit 54 such that the EGR rate is lowered when theengine 10 is operated under certain condition requiring particularlysmooth and/or efficient engine operation as exemplified by a high speedlow load condition and a low speed high load condition.

There will be no need of explaining the particulars of the controlcircuit 54 since analoguous electronic control circuits are well knownwith regard to feedback control of air-to-fuel ratio in intake systemsfor internal combustion engines.

The EGR control system of FIG. 1 is based on the following way ofthinking.

Some portion of the combustion gas produced at each engine cycle is leftunexhausted from the combustion chamber 12. The retained combustion gasis not different from the recirculated exhaust gas except fortemperature, so that the extent of the suppression of NO_(x) formationthrough lowering in the rate of combustion and maximum combustiontemperature depends on the total amount of the recirculated exhaust gasand the retained combustion gas. It is quite desirable, therefore, thatthe amount of the retained combustion gas too is taken into account incontrolling the rate of EGR. However, the amount of the retainedcombustion gas exhibits a great variation with changes in the engineoperating condition, and it is not easy to regulate the rate of EGR inresponse to such variation in the amount of the retained combustion gas.Accordingly this variation has been left out of consideration inconventional EGR control systems: the volume of the recirculated exhaustgas has been regulated exclusively in accordance with the volume ofadmitted air represented by the magnitude of vacuum at a certain sectionof the induction passage. It has been inevitable, therefore, that aconsiderable fluctuation occurs in the proportion of the combustion gasto the fresh gas mixture in the combustion chamber, causing anintermittent variation in the operability of the engine, with changes inthe engine operating condition even when the rate of EGR is controlledas intended.

The above described problem remains unsolved even when a feedbackcontrol is accomplished by detecting the quantity of actuallyrecirculated exhaust gas. There is a need of detecting also the volumeof the unexhausted combustion gas relative to the volume of freshmixture admitted at each intake stroke. However, it is not essentiallynecessary to detect the volumes of the recirculated exhaust gas and theunexhausted combustion gas since these two differently called gases aresubstantially of the same nature. In the control system of FIG. 1, it isintended to almost directly detect the total volume of the recirculatedexhaust gas and the unexhausted combustion gas (these two gases willcollectively be referred to as "diluent gas" for brevity) relative tothe volume of fresh mixture or the volume of the combustion chamber.Although the ionic current I is not direct indication of the volume ofthe diluent gas, the rate of combustion U as a primary factor affectingthe ionic current I is an exact indication of the volume of the diluentgas. Accordingly the control system of FIG. 1 can accomplish a precisecontrol of EGR in best accordance with actual condition of combustion, aprincipal factor in the formation of NO_(x), under every operatingcondition of the engine without the need of measuring the flow rate theexhaust gas in the recirculation passage.

The operation of the EGR control system of FIG. 1 will have already beengrasped. If the volume of the diluent gas in the combustion chamber 12,detected by the probe 56 and the detection circuit 58 and transmitted tothe control circuit 54, is larger than an intended volume, meaning thatthe intensity of the detected ionic current I is below an expectedcurrent intensity represented by a reference signal, the control circuit54 accomplishes a corrective function so as to provide a power signal tothe electromagnetic valve 50 to allow the admission of a sufficientlylarge quantity of air into the vacuum conduit 48 thereby to decrese themagnitude of vacuum applied to the actuator 38. Then the diaphragm 40deflects towards the atmospheric pressure chamber 44 with the resultthat the valve member 34 changes its position to decrease an effectivecross-sectional area of the orifice 36. Consequently a decrease occursin the volume of the recirculated exhaust gas. The oput of the controlcircuit 54 continues to fluctuate until realization of an intendedvolume of the diluent gas in the combustion chamber 12. When thedetected ionic current I is greater than the expected one, the openingarea of the control valve 32 is increased by diminishing or interruptingthe admission of air into the vacuum conduit 48 through theelectromagnetic valve 50. Thus any deviation of the proportion of thediluent gas to the air-fuel mixture in the combustion chamber 12 from anintended proportion can quickly be cancelled.

The control system of FIG. 1 is quite effective for maintaining therelative volume of the diluent gas in the combustion chamber 12 almostconstantly at a preset value, even when an abrupt and great increaseoccurs in the amount of the unexhausted combustion gas under, forexample, a decelerating condition. It is possible, therefore, tomaintain a stable combustion or stable engine operation under everyoperating condition of the engine 10 and effectively accomplish theobject of diminishing the emission of NO_(x).

FIG. 3 shows an EGR control system which is generally similar to thecontrol system of FIG. 1 but utilizes mechanical vibration of the engine10, instead of the ionic current I in the case of FIG. 1, for producinga feedback signal representing actual condition of combustion in thecombustion chamber 12.

As is commonly recognized, mechanical vibration of the engine 10intensifies as the combustion in the combustion chamber 12 tends towardsinstability as the result of an increase in the relative volume of thediluent gas. It is possible, therefore, to estimate the condition ofcombustion and the relative volume of the diluent gas in the combustionchamber 12 with accuracy from the magnitude of the engine vibration.

In FIG. 3, a vibration pickup 60 is mounted on the body of the engine10. The vibration pickup 60 is of an electric type such as moving magnettype, piezoelectric type or a strain gauge type. The output of thepickup 60 is transmitted to an electronic control circuit 54A, which isfundamentally similar to the control circuit 54 in FIG. 1, via avibration detection circuit 62. In other respects (of course the probe56 and the ionic current detection circuit 58 are excluded) the controlsystem of FIG. 3 is identical with the system of FIG. 1. The controlcircuit 54A provides a variable power signal to the electromagneticvalve 50 based on the deviation of the engine vibration signal suppliedfrom the vibration sensing means 60, 62 from a reference signalrepresenting an expected magnitude of the vibration corresponding to anintended amount of the diluent gas in the combustion chamber 12. Thecontrol of the function of the valve actuator 38 by the electromagneticvalve 50 is the same as in the control system of FIG. 1.

The performance of the control system of FIG. 3 is fundamentally similarto that of the control system of FIG. 1. As a particular advantage ofthe control system of FIG. 3, the rate of EGR can naturally be loweredwhen the engine 10 is operated under a condition which is liable tocause instability in the engine operation but does not allow theformation of a large quantity of NO_(x) as exemplified by an abruptlydecelerating condition or idling condition. A sharp increase occurs inthe mangitude of the intake vacuum during, for example, an abruptdeceleration, so that both the maximum combustion temperature and therate of combustion exhibit considerable lowering because of a decreasein the compression pressure and an increase in the amount of unexhaustedcombustion gas, resulting in a great diminishment of the generation ofNO_(x). It is desirable, therefore, that the rate of EGR be greatlylowered, even to zero, under such a condition. Since the magnitude ofthe engine vibration is detected as a feedback signal, the controlsystem of FIG. 3 can lower the rate of EGR in good response to a changefor the worse in the combustion condition under the above describedcondition.

As a still different embodiment of the invention, FIG. 4 shows an EGRcontrol system in which a feedback signal is produced by detecting thefrequency of misfire in the engine 10 as an indication of the conditionof combustion. When EGR is effected, an increase in the relative volumeof the diluent gas causes an increase in the frequency of misfire in theengine combustion chambers.

FIG. 5 shows the principle of a misfire sensor employed in the controlsystems of FIG. 4. A pair of needle-shaped electrodes 64 are disposed inthe exhaust passage 28 at a section 28a not far distant from thecombustion chambers (so that the exhaust gas has a sufficiently hightemperature at this section 28a) with a suitable gap between their tipsso as to provide a spark gap 65, and an appropriate voltage iscontinously impressed across these electrodes 64. The spark-gap 65 andthe impressed voltage are adjusted such that spark is produced acrossthe gap 65 while exhaust gas passing through the gap 65 is truly acombustion gas (accordingly ions are present in the exhaust gas). If theexhaust gas contains a certain amount of unburned air-fuel mixture, thespark is produced at a less frequency because of a lowering in the iondensity in the exhaust gas. No spark will be produced when the amount ofthe unburned air-fuel mixture increases to a certain level. An increasein the amount of unburned air-fuel mixture in the exhaust gas means anincrease in the frequency of misfire in the engine combustion chambers,so that the frequency of misfire can be estimated from the frequency ofthe sparking across the gap 65. The occurrence of each spark can bedetected as a spark-discharge current.

The engine 10 in FIG. 4 is provided with an exhaust manifold 66, and amisfire sensor 68 having the spark-gap 65 is disposed in each of themanifold branches 66a, 66b, 66c and 66d. However, it is permissible toprovide the misfire sensor(s) to only a portion of the manifold branches66a, 66b, 66c, 66d. Still alternatively, a single misfire sensor 68 maybe disposed in a region 67 where join the exhaust streams from all themanifold branches 66a, 66b, 66c, 66d. The output of the misfire sensors68 is transmitted to a control circuit 54B, which is fundamentallysimilar to the control circuit 54 in FIG. 1, via a misfire orspark-discharge current detection circuit 70. The output of the circuit70 takes the form of a pulse signal as illustrated by a model of FIG. 6(assuming that a variation occurs in the amount of unburned air-fuelmixture in the exhaust gas). Accordingly the control circuit 54B is soconstructed as to count the number of the current pulses per a definiteamount of time. Alternatively the current of the pulses may beintegrated over a definite amount of time.

In the control system of FIG. 4, a decrease in the frequency of thesparking means an increase in the misfire frequency and hence anincrease in the relative volume of the diluent gas in the combustionchambers. Then the control circuit 54B varies its output so as to lowerthe rate of EGR through the operation of the electromagnetic valve 50which regulates the magnitude of vacuum applied to the control valve 32.This control circuit, therefore, accomplishes a precise feedback controlof EGR, or the amount of the diluent gas, regardless of the operatingcondition of the engine 10 with good responsiveness and precisionrequired for precluding a substantially unstable engine operationsimilarly to the control systems of FIGS. 1 and 3.

What is claimed is:
 1. A feedback control system for controlling thevolume of exhaust gas recirculated from an exhaust passage of aninternal combustion engine to the induction passage of the enginethrough an exhaust recirculation passage, the system comprising:a flowcontrol valve to vary the volume flow rate of the exhaust gas in theexhaust recirculation passage; a sensor means for sensing the conditionof combustion in a combustion chamber of the engine and producing afirst electrical signal representing the sensed combustion condition,said sensor means comprising a spark-gap disposed in the exhaust passageto detect the frequency of misfire in said combustion chamber by thefrequency of a spark-discharge across said spark-gap; a control meansfor producing a second electrical signal based on said first electricalsignals, said second electrical signal indicating a decrease in thevolume flow rate of the exhaust gas in the recirculation passage whensaid first electrical signal implies a decrease in the frequency of saidspark-discharge in said combustion chamber; and an actuator means foroperating said control valve in response to said second electricalsignal.
 2. A feedback control system as claimed in claim 1, wherein saidactuator means comprise a vacuum-operated actuator for operating saidcontrol valve connected to the induction passage by a vacuumtransmission passage such that the opening area of said control valveincreases as the magnitude of vacuum transmitted to said actuatorincreases and an electromagnetic valve arranged to admit a variablequantity of air into said vacuum transmission passage in response tosaid second electrical signal.
 3. A method of controlling therecirculation of a portion of exhaust gas through an internal combustionengine, comprising the steps of:detecting the condition of combustion ina combustion chamber of the engine by detecting the frequency of misfirein said combustion chamber to produce an electrical first signalrepresenting the detected condition; producing an electrical secondsignal based on said first signal, said second signal indicating adecrease in the volume of the recirculated exhaust gas when said firstsignal implies that the detected combustion condition approaches anunstable condition corresponding to an increase in said misfirefrequency; and varying the volume of the recirculated exhaust gas basedon said second signal.
 4. A method as claimed in claim 3, wherein saidfrequency of misfire is detected by detecting the frequency ofspark-discharge across a spark-gap disposed in an exhaust passage of theengine at a section where the exhaust gas has a sufficiently hightemperature, said second signal indicating said decrease when said firstsignal implies a decrease in the frequency of said spark-discharge.